Nanoconfinement facilitates reactions of carbon dioxide in supercritical water

The reactions of CO2 in water under extreme pressure-temperature conditions are of great importance to the carbon storage and transport below Earth’s surface, which substantially affect the carbon budget in the atmosphere. Previous studies focus on the CO2(aq) solutions in the bulk phase, but underground aqueous solutions are often confined to the nanoscale, and nanoconfinement and solid-liquid interfaces may substantially affect chemical speciation and reaction mechanisms, which are poorly known on the molecular scale. Here, we apply extensive ab initio molecular dynamics simulations to study aqueous carbon solutions nanoconfined by graphene and stishovite (SiO2) at 10 GPa and 1000 ~ 1400 K. We find that CO2(aq) reacts more in nanoconfinement than in bulk. The stishovite-water interface makes the solutions more acidic, which shifts the chemical equilibria, and the interface chemistry also significantly affects the reaction mechanisms. Our findings suggest that CO2(aq) in deep Earth is more active than previously thought, and confining CO2 and water in nanopores may enhance the efficiency of mineral carbonation.


SUPPLEMENTARY METHODS
Because the PBE exchange-correlation functional cannot calculate the van der Waals interactions between graphene sheets and solutions well [1], we modeled graphene in contact with solutions by defining a potential that acts on carbon and oxygen atoms in solutions as a function of the perpendicular distance d between graphene and atoms. A benefit of using a model potential is that there is no surface chemistry or charge transfer between the model potential and solutions, so by comparing it with the more realistic stishovite-confined solutions, we can distinguish the effects of spatial confinement and surface chemistry. Using a model potential to create nanoconfinement for aqueous solutions has been adopted in many previous studies (e.g., [2][3][4]). The graphene-oxygen interaction was fitted to the diffusion Monte Carlo calculations for water molecules with the two-legged configuration adsorbed on the graphene sheet [5]; we put the molecular force on oxygen atoms. The interaction has the Morse potential shape (see Supplementary Fig. 1): where D O e = 9.55185 kJ mol −1 , a O = 1.34725Å −1 , and d O 0 = 3.37265Å [5].
We tried several force potentials to fit the graphene-CO 2 interaction using the optB86-vdW functional reported in [6] (see Supplementary Fig. 1), and found that the Morse potential best reproduces the interaction, where D CO 2 e = 21.50492 kJ mol −1 , a CO 2 = 1.18449Å −1 , and d CO 2 0 = 3.25282Å. The graphene-carbon interaction was then obtained by However, this gives a nonphysical attractive potential at short distances (<3Å), as shown in Supplementary Fig. 1(b). To correct this, we added the following repulsive term to the S2 graphene-carbon interaction: where D rep e = 18.69565 kJ mol −1 , a rep = 26.09666Å −1 , and d rep 0 = 2.56637Å. The overall graphene-carbon interaction is as shown in Supplementary Fig. 1(b). There is no interaction between graphene and hydrogen atoms in our simulations. We implemented the interaction Eqs. (1,5) in the Qbox code.
To study graphene-confined solutions, we modeled two model graphene surfaces in the xy plane. Water and CO 2 molecules were inserted between these sheets. There is 7Å thick vacuum, so the confined solution is at least 10Å away from its replica with periodic boundary conditions. For the convenience of comparison, the x-and y-dimensions of the unit cell are the same as those with stishovite confinement . The simulation details are shown in Supplementary Table I. At ∼10 GPa and 1000 K, we also doubled the number of molecules in the unit cell and added the van der Waals corrections [7] to test our computational setups.
We used the SG15 Optimized Norm-Conserving Vanderbilt (ONCV) pseudopotentials with a plane-wave cutoff of 65 Ry to speed up calculations [8,9]. Supplementary Fig. 4 and Supplementary Table IV show that our simulation sizes in Supplementary Table I are big enough, and the van der Waals corrections change the chemical speciation little.
We calculated the lateral pressure in the simulation cell: P = σ xx + σ yy /2. The diagonal elements of the computed stress tensor, σ xx and σ yy , are modified to account for the vacuum in the unit cell according to σ i = σ i (L z /h z ), where L z is the z-dimension of the unit cell, and h z is the distance between graphene sheets. This method was used in previous studies [3].
For stishovite-confined solutions, the stishovite slab is made by three stoichiometric layers S3 of SiO 2 exposing the low-energy (100) surface [10] to solutions, as shown in Supplementary  Table I summarizes the simulation setups.
To establish whether solutions are acidic or basic, acidity can be quantified using